LED print head imaging devices have replaced traditional ROS laser systems, enhancing cost savings and addressing reliability and uniformity issues. LED bar-type print head imaging apparatus and systems may include a print bar imager assembly having an array, usually linear, of individual sources. A print bar may comprise an array formed of smaller sub-arrays arranged side-by-side.
A print “bar” as used in this document means a structure or device holding an arrangement of light emitting diode (“LED”) print heads that remains stationary during printing. For print bars or print heads, the LED bar is the current state of the art. A lens mechanism such as a rod lens array, commercially available under the trademark SELFOC, can be used in the print bar for focusing the light emitted by the LED or LED array on the photosensitive recording member such as a photoreceptor (P/R) medium.
Due to limitations and tolerances of the lens mechanism, the depth of focus of a SELFOC lens is very small. Depth of focus is the tolerance in which either the light source, the SELFOC lens, or the photoreceptor can have a positional error (about ±60 μm) with respect to the other two components without losing the focus. Moving out of this focus range results in imaging defects. Maintaining this mechanical tolerance (about ±60 μm) may require adjustment due to production variations and environmental changes or wear over life. This constant adjustment adds to design and production cost. Various techniques have been proposed to address the so-called depth of focus problem in electrophotographic printing. Depth of focus correction methods have included replacing the light source with a laser, changing a spot size by eliminating the lens mechanism, and software processing to change the illumination profile of the light source.
There is a need in the art for methods and systems that can economically and optimally control the position of the print bar to correct for process variations and other factors that may adversely affect the depth of focus or positional errors when forming an image on a photoreceptor medium.